In recent years, there is a need for a compact and efficient motor serving as a driving motor for an electric vehicle, a hybrid vehicle and the like. For this purpose, various kinds of motors such as a synchronous reluctance motor and an embedded magnet synchronous motor have been studied.
However, the reluctance motor, which causes a relatively large torque ripple, poses a problem of a loud noise and strong vibration when it is used as a driving motor. The noise and vibration of the motor is considered to be related to the torque ripple.
Japanese Patent Laying-Open No. 2000-152577 discloses a technology for reducing such torque ripple of the reluctance motor. In this technology, in order to reduce torque ripple, the tip surface of a salient pole of a rotor is rounded off to allow the inductance of an armature winding to be changed in the shape of a sine wave.
However, as disclosed in Japanese Patent Laying-Open 2000-152577, if the measures against torque ripple are taken by structuring the salient pole of the rotor of the motor in bilateral symmetry, average torque will be reduced. Therefore, in order to produce large torque, it is necessary to provide a larger-diameter rotor. Such a larger-diameter rotor will prevent downsizing of the motor.